It is the objective of the proposed investigations to examine the binding interaction between the antidiuretic hormone and its receptors on renal plasma membranes. We propose to characterize the ADH-binding system in terms of known biochemical entities including the protein component(s) of the plasma membrane. We also intend to relate the binding properties of calcium and magnesium in the system to the binding of ADH and to the ADH-activated adenylate cyclase of renal plasma membranes. The antidiuretic hormone-inactivating enzyme system of renal plasma membranes will be isolated and characterized with regard to its effect upon ADH-binding and ADH-activation of adenylate cyclase. Finally, the inhibitory effects which calcium exhibits in the ADH-inactivating system will be investigated by measuring the altered kinetic parameters of the enzyme and the affinity of the enzyme for calcium ions.